1. Field of the Invention
The invention generally relates to electronics, and in particular, to digital predistortion of RF power amplifiers.
2. Description of the Related Art
When an amplifier amplifies a signal that contains amplitude variations, the signal will become distorted if the amplifier does not exhibit a linear amplitude and phase transfer characteristic. This means that the output of the amplifier is not linearly proportional to the input of the amplifier. It will also suffer distortion if the phase shift introduced by the amplifier is not linear over the range of frequencies present in the signal or if the phase shift caused by the amplifier varies with the amplitude of the input signal. The distortion introduced includes inter-modulation of the components of the input signal. The products of the inter-modulation can appear within the bandwidth of the signal causing undesirable interference. They can also extend outside the bandwidth originally occupied by the signal. This can cause interference in adjacent channels and violate transmitter licensing and regulatory spectral emission requirements. Although filtering can be used to remove the unwanted out-of-band distortion, filtering is not always practical, especially when the amplifier is operates on several different frequencies.
Predistortion is a technology typically used for wireless broadband in both a mobile station and a base station. Recent air-interface standards, such as 802.16e (WiMAX), rely on the use of time-division duplex (TDD) in which the transmitter and receiver alternate operation in time. Amplifier predistortion on the transmitter of either the base station or the mobile station can be used to improve the spectral and constellation error performance at high power allowing the use of a smaller and less expensive amplifier for a given power class. A smaller amplifier will typically also consume less bias current, which improves the efficiency of the base station and lengthens the battery life of the mobile station.